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The infrared vibrational modes of sol-gel derived optical silica monoliths depend upon thermal treatments and chemical environments, as shown by Fourier transform infrared spectroscopy (FTIR). A semi-empirical quantum mechanical theory (PM-3 in MOPAC 6.1) is used to analyze the structural changes responsible for the spectral shifts. Optimized structures of 2-member, 3-, 4-, 5- and 6-member rings of SiO2 are calculated. The force constants for the molecular bonds in the rings are obtained and converted to the associated vibrational spectra for the rings. The peak position of the asymmetric transverse optical (AS1TO) mode of the rings shifts from 1070 cm−1 for 2-member rings to 1100 cm−1 for 3-member rings, 1150 cnr1 for 4-member rings, 1140 cm−1 for 5-member and 1120 cm−1 for 6-member rings. The IR data show a 38 cm”1 shift of the AS1TO mode as the gel-silica density changes from 1.1 g/cc to 2.2 g/cc. Thus, the intensification and shift of the AS1TO mode in the gel-silica to higher wave-numbers corresponds to a change in the distribution to larger silicate size rings.

A three-dimensional transient model for heat conduction in silica glass is developed. The model simulates a three-dimensional temperature distribution in a silica glass irradiated by a moving CO2 laser. Both the reflectivity of the glass surface and the strong attenuation of the laser energy in the glass medium are accounted for by a detailed radiation analysis. The energy absorbed by the glass is determined to be confined in a 10 μm thickness; the laser irradiation is thus treated as a boundary condition. The heat diffusion equation is solved by an alternating direction-implicit method.

A three-dimensional transient thermal modeling of laser-densified gel-silica microlenses is described. This model simulates the three-dimensional temperature distribution in a porous gel-silica glass irradiated by a CO(2) laser. The heat induced by the laser gives rise to a gradient in temperature, which creates a gradient in density and index of refraction. The laser-densified region functions as a microlens. The modeling results include a temperature distribution with time within a volume of 2 mm × 2 mm × 2 mm. The calculated temperature distributions compare well with the measured property distributions of the microlens including microhardness and the peak positions of the Si-O-Si stretching vibration mode in IR spectra across the lens.